Mass flow control has been one of the key technologies in semiconductor chip fabrication for forty years. As the technology of chip making has improved so has the demand on the mass flow controller (MFC). Initially chip manufacturing processes were courser than today. Process steps were longer and focus was on the control of steady state flow accuracies and repeatability. One could divert the gas flow or wait to strike a plasma until the system had stabilized at steady state and little attention was paid to the transients that occurred at the turn on or off of the gas flow.
Problematically, as more refined manufacturing processes evolved with time, higher performance was needed from conventional thermal and pressure based MFCs. The stability and accuracies of the past devices were bottlenecking semiconductor fabrication process. Process step durations shortened to 5 second steps seen now verses 30 minutes process steps of the past. The relatively long transient time to change gas flow rates to the process once acceptable with the longer process steps is problematic with the shorter process steps. Further, MFCs are lagging to meet the demand for controlling gas flows over a wider flow range with more accuracy and less costly hardware.
The pressure based MFC was introduced in the last decade and is now overtaking the use of the thermal MFC in critical etch applications. In 2002, Fugasity introduced a pressure base MFC called the Criterion. The pressure based MFC was an improvement on the thermal MFC and hence was a commercial success. However, those same forces that pushed the development of the Criterion, the demand for improved performance and reduced price, are still pushing to improve the design of the pressure based MFC.
One of the issues common to thermal and pressure based MFCs is form factor. Space is very expensive in a modern semiconductor tool. The interface connecting the MFC to the other components in a gas box has been standardized by the industry to allow interchangeability of devices such as MFC and air operated shut off valve produce by a multiple different suppliers. The dominate interface standard in the industry is based on components being 1.1″ wide. MFC's are 1.1″ wide (28.6 mm) by 4.13″ (105 mm) in length with porting and other geometry details as describe in the Semi F82-0304 specification. Similarly a second interface specification, Semi F84-0304, defines the interface geometry for air operated valve as being 1.1″ wide by 1.1″ in length square interface.
Independent of the device type or manufacturer the vast majority of components (air valves, filters, check valves, regulators, etc.) found in the gas box of a modern semiconductor fabrication tool will comply with the 1.1″ square interface. MFC and Electronic regulators will fit the 1.1″ X 4.13″ rectangular interface.
These device interchangeability issues and the resulting interface standards have had the impact of preventing spontaneous component size reductions. About every 10 to 20 years the industry has seen a new smaller standard proposed and accepted, but in time periods between these adoptions, devices are, as a practical matter, forced to retain the external envelope defined by the standards.
However, internal device design improvements that allow smaller internal components, while not affecting the external envelope, have had the beneficial effect of allowing more instrumentation and functionality to be placed into the standard external envelope. For example a supply pressure transducer, typically a 1.1″ square interface, had been traditionally place upstream of an MFC. Component size reduction of the pressure transducer and similar reduction in the MFC's internal components has allow the function of the supply pressure transducer to be integrated into the MFC thus eliminating the need for the 1.1″ square interface formerly used by the pressure transducer.
What is needed is a robust MFC having various space-saving layouts that allows additional component integration within the standard envelope and which incorporates improved design, components and new functionalities to address the transient response issues and accuracy limitations inherent in the current devices. Additionally, a layout in an MFC allows for a smaller pressure based MFC package size that allow it to fit the smaller standard square interface envelope rather than requiring the larger rectangular interface that current MFCs require.